Abstract: Origins of Abnormal Pressures in the Lower Vicksburg, McAllen Ranch Field, South Texas

By

Robert R. Berg

The Vicksburg Formation consists of an upper member of shale about 2000 ft (610 m) thick and a lower member of interbedded sandstones and shales about 4000 ft (1 220 m) thick. The entire section is abnormally pressured, and gradients range from 0.86; to 0.94 psi/ft (19.2 to 21.2 kPa/m). Pressures within the section were established by extrapolation of shut-in buildup pressures and by estimation of pressures from conductivity logs. Hydrostatic heads were then calculated and displayed in a vertical potentiometric profile. Head distributions suggest that hydrodynamic flow is taking place from areas of high pressure to an underlying major, listric normal fault and then updip along the fault plane. There is also upward flow from Jackson Shale below the fault. The top of abnormal pressures occurs at a depth of 7600 ft (2286 m) and at a temperature of about 210° F (99° C) where there is an abrupt decrease in smectite within the mixed-layer illite-smectite clays. Pressure increase with temperature does not follow isodensity lines for water as in the case of aquathermal pressuring. Therefore, it is concluded that abnormal pressures are largely the result of clay transformation below a depth of 7500 ft (2286 m), perhaps accompanied by pressuring due to hydrocarbon generation below 11,000 ft (3353 m).

A second zone of abnormal pressures with gradients to 0.74 psi/ft (16.7 kPa/m) occurs at about 6000 ft (1 829 m) in the lower Frio Formation. In this zone, pressure increase with temperature follows isodensity lines for water, and it is concluded that aquathermal pressuring is the major cause of abnormal pressures. Shale densities suggest that nonequilibrium compaction may have played a minor role in creating abnormal pressures in the Frio.

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